Radiological risk assessment of use of phosphate fertilizers in soil

Abstract

We live in a milieu of radiation and exposed to ionizing radiation from natural sources. Natural radioactivity is wide spread in the earth's environment and it exists in various geological formations in soils, rocks, plants, water and air (1-3). Naturally occurring radioactive materials (NORM) became the focus of regulatory interest with the publication of International Atomic Energy Agency SS115 and the subsequent publishing of European Council Directive 96/29/EURATOM. It has changed the profile of radiation protection completely, increasing the regulatory awareness of natural radiation and the industries involved. Three major industries have been identified in terms of their scope and the materials handled as industries requiring further attention. They are the Oil & Gas Industry, the Zircon Industry and the Phosphate & Fertilizer Industry. The Phosphate and associated Fertilizer industry has an added complication, because it has two distinct sources of raw material, e.g. being of igneous or sedimentary origin. The source material contains isotopes from the natural uranium and thorium decay chains that may or may not follow the gypsum in the process. The weathering of the parent igneous rock released the radionuclides by the process of leaching and finally they entered the appetite structure by adsorption and co-precipitation Background: The radiological impact of the use of phosphate fertilizers in soil is due to the internal irradiation of the lung by the alpha particles, short lived radon-thoron progeny and the external irradiation of the body by gamma rays emitted from radionuclides in situ. This paper describes the results of gamma spectrometric measurements of the concentration of the natural radionuclides namely Ra, Th and K in the soil samples collected from the fields where a variety of phosphate fertilizers are being used by the farmers to enhance the crop yield. Materials and Methods: The experimental work utilizes actual measurements of Ra, Th and K using gamma spectrometry and radon concentration and exhalation rates measurements using solid state nuclear track (LR-115, Type-II plastic) detectors to asses a first order exposure risk for the persons working in the fields where lot of fertilizers are being used to enhance crop yield in terms of occupational exposure. Results: The concentration of Radium, Thorium and Potassium in the mixed soil sample from crop fields is 16.2 ± 0.22, 68.1±1.44 and 875.0±9.68 Bq/kg, where as in barren soil sample is 9.1±0.13, 59.4±1.45 and 668.4±8.01 Bq/kg respectively. The radium equivalent activity (Raeq) in the mixed soil sample from crop fields is 225.9 Bq/kg, where as in barren soil sample is 193.1 Bq/kg. The values of absorbed dose and annual effective dose (indoors and outdoors) are found to vary from 90.87 nGyh to 119.71 nGyh, 0.45 mSv/y to 0.59 mSv/y and 0.11 mSv/y to 0.15 mSv/y respectively in soil sample from crop fields, whereas the value of absorbed dose and annual effective dose (indoors and outdoors) is 92.29 nGyh, 0.45 mSv/y, 0.11 respectively in soil sample collected from barren land. The radon concentration and exhalation rates have also been reported. Conclusion: The activity concentration, exhalation rate and absorbed dose were found to increase substantially with the use of phosphate fertilizers and it varies from sample to sample. The radium equivalent activities in all the soil samples were found to be lower than the limit (370 Bq/kg) set in the Organization for Economic Cooperation and Development (OECD) report and the dose equivalent is within the safe limit of 1 mSv/y. Iran. J. Radiat. Res., 2006; 4 (2): 63-70